1. Field of the Invention
This invention relates to an image pickup apparatus having an autofocusing function, and more particularly, to an autofocusing (hereinafter termed "AF") function of an electronic still-picture camera.
2. Description of the Prior Art
FIG. 1 is a diagram showing the configuration of an electronic still-picture camera having an autofocusing function which detects the amount of defocus of an image from an output value from a solid-state image pickup device for photographing an object, and automatically performs a focus operation based on the detected value. In FIG. 1, reference numeral 1 represents a lens unit including a focusing lens. Lens driving motor 2 adjusts the position of lens unit 1. Reference numeral 3 represents a diaphragm. Diaphragm driving circuit 4 adjusts diaphragm 3. Solid-state image pickup device 5 converts an image of an object (optical image) focused by lens unit 1 into an electric signal. Solid-state image pickup device driving circuit 6 drives solid-state image pickup device 5. A/D conversion circuit 7 performs A/D (analog-to-digital) conversion of the output of solid-state image pickup device 5. Memory (frame memory) 8 stores the output of A/D conversion circuit 7 as still-picture image data. ES filter 9 calculates an ES value (to be described later) representing the amount of defocus based on an image signal read from memory 8. System control circuit 10 includes a CPU (central processing unit), a programable ROM (read-only memory), a RAM (random access memory) and the like, and controls the entire system.
Image-signal processing circuit 11 performs processing, such as .gamma.-conversion, band limitation and the like, on the output of memory 8. D/A conversion circuit 12 performs D/A (digital-to-analog) conversion of the output of image-signal processing circuit 11. FM modulation circuit 13 performs FM modulation of the output of D/A conversion circuit 12. Recording amplifier 14 performs current amplification of the output of FM modulation circuit 13. Recording head 15 is connected to recording amplifier 14. Reference numeral 16 represents a magnetic sheet (floppy disk), serving as a recording medium. Motor 17 rotates magnetic sheet 16. Motor servo circuit 18 stabilizes the rotation of motor 17. Photometric device 19 measures the brightness of the object. By depressing shutter-release switch 20, a series of photographing operations are started.
FIG. 2 is a diagram illustrating an interline-transfer-type solid-state image pickup device which has been frequently used as the above-described solid-state image pickup device 5. In FIG. 2, reference numeral 501 represents the entire interline-transfer-type solid-state image pickup device. Each photodiode 502 converts light into electric charges and stores them. Each vertical CCD (charge-coupled device) 503 transfers electric charges generated by photodiodes 502 in the vertical direction. There are also shown transfer electrodes V1-V4 for vertical CCD's 503. Electrode V1 also functions as a transfer gate to transfer electric charges in odd-column photodiodes 502, and electrode V3 also functions as a transfer gate to transfer electric charges in even-column photodiodes 502. Vertical CCD's are driven, for example, with four-phase transfer pulses. Horizontal CCD 504 transfers electric charges transferred from vertical CCD's 503 in the horizontal direction. Transfer electrodes H1 and H2 for horizontal CCD 504 are driven, for example, two-phase transfer pulses. Output amplifier 505 converts electric charges into a voltage and outputs the voltage. Top drain 506 sweeps away unnecessary electric charges by transferring them in the reverse direction. Bottom drain 507 sweeps away unnecessary electric charges by transferring them in the forward direction.
FIG. 3 illustrates an operational sequence of the electronic still-picture camera having an autofocusing function shown in FIG. 1. When shutter-release button 20 has been depressed (turned on or closed) at time T0, a series of photographing operations are started. System control circuit 10 calculates the optimum stop value Av and the optimum shutter speed Tv for a photographing operation in accordance with the output of photometric device 19. Next, system control circuit 10 sets diaphragm 3 to an open state between times T1 and T2, and makes solid-state image pickup device 5 perform a series of operations n times, i.e., sweeping-away of unnecessary electric charges, exposure, and reading of signal electric charges, and moves lens unit 1 in n steps or continuously from an infinite position to the nearest position through a focus position using lens driving motor 2. That is, n times of reading operations of signal electric charges from solid-state image pickup device 5 are performed, and the amount of defocus is calculated and lens unit 1 is moved based on a signal read at each reading operation. Thus, the lens position having the minimum amount of defocus, that is, the optimum focus position or lens position P in an in-focus state is found. The exposure time at this AF operation is set so that the amount of exposure during the AF operation equals the amount of exposure obtained with the above-described optimum stop value Av and the optimum shutter speed Tv. Thereafter, system control circuit 10 sets the stop value of diaphragm 3 to the above-described optimum stop value using diaphragm driving circuit 4, and moves lens unit 1 from the nearest position to lens position P in an in-focus state using lens driving motor 2 between times T3 and T4. Then, at time T4, system control circuit 10 performs a clearing operation for solid-state image pickup device 5 using solid-state image pickup device driving circuit 6, wherein unnecessary electric charges are swept away into top drain 506 by transmitting them in the reverse direction. Then, photographic exposure is performed during from T4 to time T5. Thereafter, system control circuit 10 performs reading of signal electric charges from time T5, and records the processed signal on magnetic sheet 16.
FIGS. 4(a) through 4(f) are diagrams illustrating the ES method, which serves as a method for detecting the above-described amount of defocus. Since the ES method is a known technique disclosed in U.S. Pat. No. 4,804,831, only a brief description thereof will be provided. FIG. 4(a) illustrates image signals. The edge of the signal is steep in an in-focus state, and is less steep in an out-of-focus state. FIG. 4(b) illustrates the differentiated waveforms D of the respective image signals. FIGS. 4(c) and 4(d) illustrate delay signals DL1 and DL2 of the differentiated waveforms, respectively. FIG. 4(e) illustrates the integrated waveforms I of the image signals, representing the contrast of the edge portions of the respective image signals. As shown in FIG. 4(f), the ES value is obtained by dividing the data of the differentiated waveform D by the data of the integrated waveform I, and represents the sharpness of the edge of the signal.
FIG. 5 is a diagram illustrating an example of the configuration of ES filter 9 shown in FIG. 1. In FIG. 5, there are shown differentiating circuit 901, absolute-value circuit 902, delay circuit 903, integrating circuit 904, division circuit 905 and peak-holding circuit 906. Using this ES filter 9, the peak value of the ES value within image information for one picture frame is represented as the ES value of the one picture frame.
FIGS. 6(a) and 6(b) illustrate the relationship between the lens position and changes in the ES value when an AF operation is performed for the purpose of obtaining the focus position. The lens of lens unit 1 is continuously fed from the minimum position to the maximum position by lens driving motor 2. At that time, image information stored in solid-state image pickup device 5 is read for every vertical scanning period (abbreviated "1V"), each ES value is obtained from the read image information for each 1V, and the lens position having the maximum ES value is made to be the focus position. A curve having a peak at the focus position depicted when the abscissa represents the amount of lens feed or time and the ordinate represents a focus signal (the ES value in the present embodiment) is termed a mountaineering curve. Since the mountaineering curve in the ES method is steep, accuracy in focus detection is high. In the foregoing description, an AF operation is performed using the entire information within each picture frame of input image information. Usually, however, a part of a picture frame is assigned as a range frame, and an AF operation is performed using only a signal within the range frame, since promptness is required for an AF operation.
In the above-described conventional approach, however, the amount of exposure during an AF operation is set so as to be optimum during a photographing operation from time T4 to time T5. Such a value is not necessarily the optimum amount of exposure for the AF operation. In addition, when a high-brightness portion, such as a light source or the like, is present within the range frame, focus information as if the lens were in an in-focus state is obtained even in an out-of-focus state. As a result, an exact AF operation cannot be performed.